The extracorporeal blood circuit of known blood treatment apparatuses comprises a device for separating air, generally also referred to as an air separator, in addition to the arterial and venous blood line, and the blood chamber of the dialyser or filter. The air separator has a chamber through which blood flows. In the process, the chamber of the air separator is not completely filled with blood. A monitoring device is used to monitor whether the chamber is filled with blood. Known drip chambers, arranged in the venous blood line downstream of the dialyser or filter, are used as air separators.
In known blood treatment apparatuses, a line generally branches off from the venous drip chamber, by means of which line the fluid system of the blood treatment apparatus can be ventilated during the filling of the extracorporeal blood circuit. The ventilation line is closed during the blood treatment.
In principle, there is a risk of the blood level in the venous drip chamber rising to the extent that blood enters the ventilation line during the extracorporeal blood treatment. As a safety measure, at least one hydrophobic filter, which is impermeable to fluid but permeable to air as long as the filter is not wetted by fluid, is arranged in the ventilation line.
Thus, the hydrophobic filter can effectively retain the blood, even if blood should enter the ventilation line. However, it is disadvantageous that the filter has to be cleaned or replaced after it has been wetted by blood, which results in an interruption of the blood treatment and constitutes a considerable amount of effort.
Before or after the dialysis, the extracorporeal blood circuit can be rinsed, for example using a sodium chloride solution or substitute (permeate of the dialysis fluid). In this case, there is also a risk of substitute fluid from the venous drip chamber reaching the hydrophobic filter via the ventilation line.
It may be possible to intervene in the machine control in the case of a fault. By way of example, the blood pump could be halted and the venous tube clamp could be closed. If no intervention is undertaken in the machine control in the case of a fault, there is, in principle, the risk of the membrane of the hydrophobic filter being pierced. It is possible that only one hydrophobic filter is present and it can be located in the ventilation line outside of, or, further down the line, also within, the machine. The risk of fluid entering the ventilation line can be reduced if two or more hydrophobic filters, connected in series, are arranged in the ventilation line. While the first hydrophobic filter, which can be arranged outside of the housing of the blood treatment apparatus, can still be cleaned or replaced in a relatively simple fashion, cleaning or replacing the second hydrophobic filter however requires the housing of the blood treatment apparatus to be opened.
EP 0 330 761 A1 describes a method for monitoring the fluid system of an extracorporeal blood treatment apparatus in which the pressure in the ventilation line in which a hydrophobic filter is arranged and which branches off from the venous drip chamber is monitored. The pressure is monitored in the section of the ventilation line remote from the drip chamber. In the process, periodic pressure variations are monitored which are generated by the blood pump and propagate across the tube system via the hydrophobic filter. Should blood enter the ventilation line, the membrane of the hydrophobic filter is wetted by fluid. As a result, the membrane of the hydrophobic filter becomes impermeable to air and so the periodic pressure variations can no longer spread out across the membrane of the hydrophobic filter. Therefore, the absence of periodic pressure variations is an indicator for a fault.
Should the natural pressure variations caused by the design of the peristaltic blood pump not be sufficient, EP 0 330 761 A1 proposes to amplify the natural pressure variations and generate artificial feed rate variations of the blood pump, for example by changing the pump rotational speed.